The objectives of this study are: 1) to continue investigation of biochemical and biophysical factors which determine cellular deformability of normal human erythrocytes, with particular focus on the role of cellular ATP and Ca ion concentrations, oxygen tension, pH and hormones; 2) to continue exploration of specific mechanisms leading to decreased deformability both in hemolytic states having reduced RBC life span associated with increased ion permeability and in the senescent erythrocyte; 3) to characterize the capillary flow properties of normal and pathologic human erythrocytes in in vitro glass microcapillaries and in in vivo mesenteric, skeletal and cardiac muscle capillaries of animals; and 4) to investigate mechanisms contributing to stasis and reversal of stasis in in vivo capillaries particularly in the case of sickle cells, and senescent cells. Studies will include separation of erythrocyte membrane proteins with characterization of divalent cation binding and calcium-induced protein conformational changes; characterization of Ca ion binding to inner surface of erythrocyte membranes as a function of ghost ATP and effects of binding on membrane ion permeability, deformability and distensibility; determination of cellular deformability and membrane viscoelastic properties by glass micropipette and polycarbonate filter technique; and observation of capillary flow by microscopic techniques coupled with videotape and standard color film recording. Measurements of capillary intraluminal pressures will be made via micropipettes by sensitive variable reluctance transducers and pH and pO2 will be monitored by 02 and pH microelectrodes. Electron microscopy will be used to assist in characterization of membrane proteins and to define pathologic cell-capillary interactions.